Radiation and chemical mutagens are known to produce quite different kinds of changes, the first being responsible for chromosome breaks and gross damage to the genetic material whereas the second produces more gene mutations, particularly those that result in fractional or mosaic individuals. Tests are being carried out on the hypothesis that some of these differences may be the secondary result of lethality caused by radiation. These tests involve the simultaneous treatment of Drosophila males with radiation and a chemical mutagen (DEN) which is known to produce primarily fractional mutants. Two different kinds of mutants are currently under investigation: visible mutations at the dumpy locus, and sex-linked lethals of the non-autonomous type.